Crimped binding strap and method of making same



Nov. 8, 1966 F. R. CRAMTON 3,

CRIMPED BINDING STRAP AND METHOD OF MAKING SAME Filed Aug. 7, 1964 INVEN 1 OR F1626 BY 1 W ATTORNEYS FRANK R. GRAMTON United States Patent 3,283,378 CRIMPED BINDING STRAP AND METHOD OF MAKING SAME Frank R. Cramton, Burlington, Vt, assignor to E. B. & A. C. Whiting Company Filed Aug. 7, 1964, Ser. No. 388,179 14 Claims. (Cl. 24-16) This application is a continuation-impart of my application Serial No. 268,796, filed March 28, 1963, now abandoned.

This invention relates to'an improved thermoplastic strap having a novel configuration which increases the straps energy to break, elongation, and resistance to splitting as well as the ease with which it is adapted to sealing, clamping, or otherwise banding boxes, packages, and the like. More particularly, this invention relates to an improved strap made from an isotactic thermoplastic polymer having a sinusoidal longitudinal configuration.

It is well known that the most widely used strapping material, today, is steel. Steel strapping inherently possesses undesirable features since it tends to rust, it co-rrodes when exposed to certain chemicals and it is difiicult and dangerous to handle. In order to overcome the foregoing problems, a number of non-metallic straps have been introduced. However, these, too, possess certain disadvantages. For instance, the prior art non-metallic straps are unable to maintain a good seal and tend to split longitudinally.

The foregoing described disadvantages are avoided by the practice of this invention which, briefly, involves producing a strap having a crimped sinusoidal longitudinal configuration and comprising a longitudinally oriented isotactic thermoplastic polymer. Preferably, the strap is composed of a thermoplastic composition comprising an admixture of a longitudinally oriented thermoplastic polymer, such as isotactic polypropylene, and a rubbery elastomer, such as polyisob-utyluene.

The crimped strap of this invention overcomes the 7 previously mentioned disadvantages in that it is easy to handle and does not corrode or rust. The sinusoidal crimped configuration makes it possible to maintain an excellent frictional seal and, even more significantly, increases its resistance to longitudinal splitting and improves its elongation without loss of ultimate tensile strength. The resistance to longitudinal splitting is increased up to 50% as a result of the crimped sinusoidal configuration. Splits that do occur tend to be limited to a single crimp rather than continuing along the length of the strap. "films, the strap exhibits resistance to such continued splitting even after a split has started. As previously mentioned, the crimped strap possesses excellent elongation. A level or flat thermoplastic polymer strap generally has an elongation of approximately 10 per cent whereas the crimped strap possesses an elongation in the range of from about 30 to about 80% depending upon the constituents contained in the composition of the strap. This increase in elongation is unexpected since the crimps should theoretically add only about 2% increase in level length per length of crimped strap. The elongation is primarily elastic in nature up to the point where the strap has been pulled and flattened. Such behavior enables the crim-ped strapping to absorb much greater shock load energy without breaking when packages strapped therewith are dropped or thrown about than when bound with other types of prior art strapping. The energy required to break the strap is known as energy-to-break.

The crimped strap of the present invention is produced by preparing a melt of the thermoplastic polymer composition, melt extruding a flat strap having a previously determined cross-sectional dimension, solidifying the strap, heat softening and longitudinally stretching the strap to orient the molecular structure, subsequently crimping the 3,283,378 Patented Nov. 8, 1966 ice strap into the form of a longitudinal sinusoid, and cooling to solidify the polymer composition. Prior to the heat softening step, the extruded strap may be quenched in a non-solvent bath such as water. While the strap has a longitudinal configuration resembling that of a sinusoidal curve, the transverse section is maintained at -a substantially constant cross-sectional thickness even though the transverse extending portions may form an oblique section.

Thermoplastic polymers which may be used in the practice of this invention include the high molecular weight (i.e., above about 45,000) solid polymers exhibiting a crystalline X-ray diffraction pattern. Polyolefin polymers such .as polypropylene and polyethylene may be employed. Polypropylene is preferred. Among the other solid polymers which may be employed are polyesters, polycarbonates, polyamides, acrylic resins and many of the other polymeric materials.

The preferred polypropylene which may be used in the practices of this invention is the isotactic form which has a density of between 0.90 and 0.94, a melt flow of from 2.0 to 3.5, and :a melting pint above 320 F. Such a polymer may be prepared by methods now well known in the art as described by G. Natta in the Journal of Polymer Science, volume XVI, pages 143 to 154 1955) and in US. Patents 2,882,263; 2,874,153 and 2,913,442.

In addition to a strap which consists of one of the foregoing thermoplastic polymers such as isotactic polypropylene, the invention contemplates the use of thermoplastic compositions comp-rising an admixture of said polymer and at least one rubbery elastomer. The term rubbery elastomers encompasses both the natural and synthetic rubbers. Included within this group are the butadiene-styrene, polyurethane, bntyl rubber, polyisolbutylene, :isoprene, polychloroprene and silicone rubbers. The admixture contains from about 1.0% to about 30% by weight of the rubber elastomer, preferably about 10% polyis-obutylene.

The crimped strap of this invention is made according to the following process. The polymer or polymer composition is introduced into the heating zone of an extruder to form a melt of the polymer which is then extruded into a flat band or strap like filament. After the strap has been extruded, it may .be solidified as by quenching in a non-solvent bath or by air-cooling. The strap is heat softened by some suitable means. This may be accomplished by conducting it througha heating chamber wherein it is supported, preferably, by a plurality of rolls or equivalent supporting members, the peripheral surface of each providing a supporting surface. The strap is heated while in the heating zone by means of a fluid source of heat, either gas or liquid, or by means of independently heated rolls. Alternatively, the supporting rolls may be eliminated from the heating zone and the strap passed directly through a zone which is heated by some fluid source of heat, or by radiant heat, the filament being unsupported by any member providing .a supporting surface which is located within the heating zone. The softened strap may be snubbed by means of a suitable device such as a conventional snubbing roll assembly after it leaves the heating chamber but prior to the drawing operation. Alternatively, the strap may also be heated and drawn in the same zone with the snubs placed outside the heating zone. The softened strap is then stretched longitudinally to increase the molecular orientation along the fiber axis. Any amount of stretching will increase the molecular orientation, however, maximum benefits are attained by stretching the strap from about 6 to about 11 or more times of its length. The ratio of 8:1 is preferred.

After the strap is stretch oriented and while still in a. softened condition, it is crimped into the form of a longitudinal sinusoid having a substantially constant thickness. The crimped strap may contain fro-m about two to about twelve full waves per inch, depending on the thickness of the material. The amplitude, which is defined as the height of the wave from the top of the crimp through this invention.

FIG. 2 is a schematic view of a section of a modification of the apparatus shown in FIG. 1.

FIG. '3 is a side view of the wheels which are used to sinusoidally crimp the longitudinally oriented strap.

FIG. 4 is a perspective view of one embodiment of the crimped strap showing constant thickness in the transverse section. FIG. 5 is a perspective view of another embodiment of the crimped strap showing constant thickness in the oblique section.

FIG. 6 is an enlarged partial perspective view showing an article of manufacture surrounded by the strap of the present invention and means for binding the strap.

In FIG. 1, a hopper 10 contains pellets 11 of an admixture of crystalline isotatic polypropylene and polyisobut- 1 ylene. The pellets 11 may be preheated in the hopper 10, if desired. From the hopper 10 the pellets are conveyed through the heated extrusion chamber 12 by means of a screw to a heated extrusion head 13 which contains an extrusion die 13a. In the extrusion chamber 12,. the temperature ofthe mixture is raised to above the melting point of the polypropylene-polyisobutylene composition.

It is extruded through suitable shaped orifices in the extrusion die 13a into one or more straps 14. A preferred extrusion temperature for the composition is about 480 7 F. when the straps 14 are extruded at a linear rate of from about 18 to about 50 feet per minute from the orifices.

In making straps it is advantageous to quench the extruded straps in order to solidify. them. This is accomplished, as shown in FIG. 1, by placing a quench bath "15 between the extrusion head 13 and the heating chamber 22. The extruded straps 14 are immersed in the quench bath 15, containing a liquid non-solvent for the composition, e.g., water, by a guide roll 16. The bath 15 is maintained in a suitable tank 17 at a temperature sufficient to cause solidification, for instance 60 F. An immersion time of from about 8 seconds to about seconds of the straps 14 in the bath 15 is generally sufiicient. The straps 14 are transported around a stationary pin 18 in the quench bath 15 and then over the roll 19 and into the hot air conditioning oven 22. In the oven, the extruded straps are transported over a, number of rolls 20, which may be heated, in a sinuous or zig-zag path, as heated air is circulated from overhead. As the straps pass through the heating zone, each succeeding driven roll 20 over which the straps 14 pass is driven at a slightly increased peripheral speed from that of the preceding roll so as to prevent the straps from. sagging appreciably.

The primary purpose of the series of driven r0lls 20 is to provide a heat exchange relationship between the straps 14 and the heated'air in the oven whereby the straps are uniformly softened by heat.

After leaving the last and uppermost driven roll 20, the filaments are snubbed with a three roll assembly 21, each roll of which is driven at about the same as or a higher peripheral speed than that of the preceding driven roll 20. A fast snub roll assembly 23 is provided just outside the oven 22 which is driven at a high peripheral speed and the filaments are stretched to about 6 to 11 times their original length. This increases themolecular orientation along the fiber axis. The drawn, oriented.

straps 14 are thereafter guided between a pair of spaced crimping wheels 25,'as are illustrated in FIG. 3, by guide roll 24. The crimped straps 14 are transported around rolls 26 located in a suitable bath 27. containing a nonsolvent cooling liquid 28, e.g., water to fix the crimped polymer and then over the roll 29 to be collected on a reel 30, supported by a frame 31.

FIG. 2 illustrates a modification of the apparatus shown in FIG; 1 in which extruded straps 14 are solidified by air cooling rather than by quenching. The straps '14 are passed over; the rolls 16' to allow them to solidify prior to being transported into the hot air conditioning oven 22.

FIG. 3 is an enlarged view of the crimping wheels 25 of FIG. 1. The fiat strap 14 is guided between the pair of crimping wheels 25 having rounded projection 25' to form the crimped sinusoidal binding strap 14' of the present invention. The crimping wheels 25 contain pro- 'jections 25 which may be parallel to the axis of the wheel 'or which may vary from parallel to helical for making straps containing transverse and oblique crimps as lil lustrated in FIGS. 4' and 5, respectively.

In FIG. 6, the sinusoidal strap 32 is positioned around an article of manufacture 34 and tightened by any suitable means in a manner similar to that adapted for steel 'band strapping. The sinusoidal ends of the sinusoidal strap overlap and intermesh to frictionally engage with one another while a metal clamp 33 having a similanconfiguration serves to maintain or urge the frictional engagement.

. The following examples illustrate the best mode contemplated for carrying out this invention.

Example 1 extruder of the type shown in FIG. 1 having a 2.0 inch screw diameter. The die plate contains extrusion orifices, each having a rectangular cross section. The jacket is heated to a temperature of about 480 F. and the straps are extruded at a linear rate of 25 feet, per minute from the orifices. The cross sections of each of the extruded straps are substantially fiat rectangles of constant thickness. The straps are then passed through a quench bath maintained at a temperature of about 60 F. to solidify them. They are then fed into a heating chamber and are transported over a series of rolls. Hot air is circulated through the heating chamber, thereby heating the straps to'a temperature of about 300 F. before leaving the heated chamber. After the heating chamber, the straps are stretched to about eight times their original length.

'The cross sections of the drawn straps are 0.375" x 0.02".

The fiat straps are then crimped by intermeshing wheels having rounded transverse projections and driven at a slightly lower speed than the draw rolls, to form the longitudinal sinusoid having about 6 /2 waves per inch and a constant transverse thickness. The crimped straps are solidified in a water bath and collected on a reel.

Example 2 Strap Size: Crimped polypropylene Energy to break (in.-lbs/in.)

Example 3 The process of Example 2 is repeated with the exception that a mechanical admixture of by weight of polyisobutylene and 90% by Weight of isotactic polypropylene having a melt flow of 2.0 at 230 C. as per A.S.T.M. designation D 123 862T is fed into the screw extruder.

The following table lists the results of tests which compare the crimped strap of Example 3 to a conventional crimped steel strap having substantially the same cross sectional dimensions. As can be seen from this table, the crimped strap of the present invention requires approximately 1.2 to 3.2 times more energy to break than did the crimped steel strap. Both straps have cross sec tional dimensions of 0.020 x 0.50 inch and contain a crimp of 3 cycles-in. with an amplitude of 0.03 inch.

CRIMPED POLYPROPYLE-POLYISOBUTYLENE STRAP VS. CRIMPED STEEL STRAP Having thus described my invention, 1 claim:

1. A flexible binding strap comprising a longitudinally stretch oriented thermoplastic polymer, said strap having a substantially longitudinal sinusoidal resiliently deformable configuration.

2. A flexible binding strap as set forth in claim 1, wherein said polymer is isotactic polypropylene.

3. A flexible binding strap as set forth in claim 1, wherein the transverse cross-sectional thickness of said strap is substantially constant.

4. A flexible binding strap as set forth in claim 1, wherein the oblique crosswsectionall thickness of said strap is substantially constant.

5. A flexible binding strap comprising a longitudinally stretch oriented thermoplastic composition, said composition comprising an admixture of a thermoplastic polymer and at least one rubbery elastomer, said strap having a substantially longitudinal sinusoidal resiliently deformable configuration.

6. A strap as set forth in claim 5 wherein said rubbery elastomer com-prises from about 1% to about 30% by Weight of said composition.

7. A strap as set forth in claim 6, wherein said rubbery elastomer comprises about 10% by weight polyisobutylene.

8. A strap as set forth in claim 5, wherein said polymer is isotactic polypropylene.

9. A strap as set forth in claim 8, wherein said rubbery elastomer is polyisobutylene.

10. A method of making a crimped flexible strap of the type set forth in claim 1, which comprises forming a longitudinally stretch oriented strap of said polymer, mechanically crimping said strap along the longitudinal axis thereof to form a resiliently deformable longitudinally oriented sinusoid and solidifying said crimped strap.

11. A method of making a sinusoidally crinrped flexible strap for use in binding packages and the like which comprises melt extruding a strap having a substantially constant cross-sectional thickness and comprising isotactic polypropylene, longitudinally stretch orienting said strap when in a softened condition, mechanically crimping said softened strap in the form of a longitudinal sinusoid, and solidifying said crimped strap.

12. A method of making a sinusoidally crimped flexible binding strap which comprises melt extruding a strap comprising an admixture of isotactic polypropylene and at least one rubbery elastomer, longitudinally stretch orienting said strap when in a softened condition, mechanically crimping said softened strap into the form of a longitudinal sinusoid, and solidifying said crimped strap.

13. An article of manufacture surrounded by a crimped flexible binding strap, said strap comprising a longitudinally stretch oriented thermoplastic polymer in the form of a longitudinal resiliently deformable sinusoid, said article being bound by frictionally engaged overlapping end portions of said strap, said end portions being urged into frictional engagement by clamping means.

14. A method of making a sinusoidally crirnped flexible binding strap which comprises melt extruding a thermoplastic polymer, longitudinally stretch orienting said strap when in a softened condition, mechanically crimping said softened strap into the form of a longitudinal sinusoid, and solidifying said crimped strap.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCES German printed application, 1,151,922, 7/1963;

WILLIAM FELDMAN, Primary Examiner. DONALD A. GRIFFIN, Assistant Examiner. 

1. A FLEXIBLE BINDING STRAP COMPRISING A LONGITUDINALLY STRETCH ORIENTED THERMOPLASTIC POLYMER, SAID STRAP HAVING A SUBSTANTIALLY LONGITUDINAL SINUSOIDAL RESILIENTLY DEFORMABLE CONFIGURATION. 